Miniature electro acoustic transducers have long been fundamental components of communications equipment ranging from telephones to hearing aids and most recently to personal listening devices such as MP3 players. In general there are two technologies available for producing such speakers, which technologies are generally referred to in the industry by the terminology “balanced armature” and “moving coil.” Conventional balanced armature technology uses two magnetic fields, one static and another responsive to a signal to produce force that moves a sound generating surface. Moving coil technology employs a single, static radially disposed, magnetic field through which a coil resides in an air gap in the radial field. When current flows through the coil in response to an electrical signal carried it the coil, a force is generated perpendicular to the plane of both the radial magnetic flux and the path of the wire coiling through the air gap. Each technology finds usefulness in particular applications. For example, in the context of audio speakers, the moving coil technology generally dominating usage with respect to larger speakers. As a moving coil speaker is reduced in size, however, the central magnetic pole residing on the shorter radius of the air gap becomes smaller and smaller, and it finally reaches a dimension where it can no longer effectively carry sufficient magnetic flux for an operable speaker. Thus, as a practical matter, moving coil speakers are seldom produced having diameters smaller than about 8 mm. On the other hand, again in the context of audio speakers, the balanced armature technology finds its greatest use in extremely small speakers such as those used for hearing aids within the listener's ear canal. The balanced armature technology has size limitations as it grows larger, because the total excursion of the sound generating surface must be within the limits of the air gap between the static poles. As a practical matter, balanced armature technologies are seldom produced having major dimensions exceeding 10 mm.
A further limitation to the performance of conventional balanced armature electro acoustic devices, (whether used as speakers or microphones) is that their frequency spectra deviate from being perfectly flat, spectral flatness being one representation of a lack of distortion, a very desirable characteristic for acoustic (and most other) transducers. This spectral deviation or “signature” arises from the fundamental structural properties that are characteristic of all conventional balanced armature devices: the mass and springiness of: the armature itself, the sound producing diaphragm and its chamber(s), and, in most conventional speaker of this type, of the connector element and its attachments that link the armature and the diaphragm. Numerous techniques have been developed to minimize the disadvantages of this inherent signature, including, for example, the use of so-called “ferro-fluids” for damping the system and improving the transducer's dynamic performance.
Notwithstanding the substantial enhancements to these general types of transducers, room remains for improving and simplifying the frequency signature, minimizing the frictional and other mechanical losses, and improving the efficiency of this type of speakers. In many applications, it also is desirable to further reduce the size of the transducer. For example, when used in a hearing aid or earphone application, it is desirable to have a transducer that is small enough to comfortably fit within a human auditory canal. Similarly, when used as a component of a device, such as a cell phone, the small size of the transducer allows the size of the device to be minimized.
Reference will be made in detail to certain exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings.